Technical Field
Embodiments of the subject matter disclosed herein generally relate to deploying and recovering tow members of a marine survey system and, more particularly, to a collar system on a tow member.
Discussion of the Background
Interest in developing offshore oil and gas production fields has dramatically increased in recent years. Due to the high cost of offshore drilling, those undertaking it rely heavily on marine surveys and other geological investigations for selecting drilling locations so as to minimize the risk of a dry well.
Marine surveys generate profiles (images) of the geophysical structure under the seafloor. While these profiles do not provide an accurate location of oil and gas reservoirs, those trained in the field may use them to estimate the presence or absence of oil and/or gas.
A marine survey may be performed using the marine seismic survey system 100 illustrated in FIG. 1 (bird's-eye view). A vessel 110 tows seismic sources 120 and multiple seismic streamers 130 through the water. The seismic sources 120 typically include plural sub-arrays of air guns configured to generate seismic waves. These seismic waves propagate downward into the geophysical structure under the seafloor and are reflected upward from interfaces between geological layers, inside which the seismic waves propagate with different speeds. Hydrophones embedded in the seismic streamers detect the reflected waves. Data related to the reflected waves is recorded and processed to provide information about the underlying geological features.
Lately, interest has increased in repeating marine surveys in the same areas at long time intervals (months or years) to monitor the evolution of geophysical structure under the seafloor (e.g., changes caused by extraction of oil and gas from a deposit). The marine survey data acquired during distinct surveys of the same area may be assembled to form four-dimensional (4D) data sets. In this context, it increases the importance of being able to accurately reproduce a baseline (i.e., earlier or first) survey in a later monitor survey(s).
Reproducing the baseline survey means reproducing the geometry of the marine survey system (i.e., relative positions of the source and detectors), as well as reproducing the location of the shots (i.e., positions at which the waves are generated). This task is difficult to accomplish because the marine survey system is subject to currents, winds, etc., and has limited maneuverability. Separation members such as (but not limited to) ropes and cables are frequently used on a marine survey system's interconnected towed components to enhance its maneuverability and geometric stability. However, the presence of these separation members may make deployment and, particularly, recovery of the towed components difficult.
In U.S. Patent Application Publication No. 2010/0170428, separation ropes are attached via sliders to umbilical cables used to tow source sub-arrays. A slider is configured to switch between an engaged state and a disengaged state. During the engaged state, the slider is locked at a fixed position along the umbilical cable. During the disengaged state, the slider moves freely along the umbilical cable.
FIG. 2 illustrates a conventional seismic source 200 consisting of two groups of sub-arrays, group 210 (including sub-arrays 212, 214 and 216) and group 220 (including sub-arrays 222, 224 and 226). Sub-arrays 212, 214, 216, 222, 224 and 226 are connected to towing vessel 201 via tow members 213, 215, 217, 223, 225 and 227, respectively. These tow members are also known as “umbilical cables.” Separation ropes 230, 232, 234 and 236 may be connected via sliders at fixed locations along tow members 213, 215, 217, 223, 225 and 227, respectively. Other separation ropes 240 and 242 may interconnect tow members 213 and 227 to lead-in cables 250 and 252, respectively.
The presence of a separation rope prevents a distance between the fixed locations along the tow members (where the separation rope's ends are attached) from exceeding the length of the separation rope. However, on one hand, the presence of separation ropes complicates and delays source sub-array recovery. On the other hand, because the mechanism keeping the slider at the fixed position is exposed for long periods to the marine environment, the mechanism may degrade undesirably and allow the slider to drift along the umbilical cable. Particularly when the towing trajectory is curved, as suggested by arrow 254, separation ropes may experience substantial tensions, causing the slider to disengage as emphasized by oval A in FIG. 2 (showing that a slider holding one end of separation rope 230 slid from its fixed location along tow member 213).
Accordingly, it would be desirable to provide mechanisms and methods that avoid the afore-described problems and drawbacks related to separation members mounted between members used to tow components of marine survey systems.